The present invention relates to an imaging apparatus and a method using the nuclear magnetic resonance (hereinafter simply referred to as "NMR"), and more particularly to an NMR imaging apparatus and method which can completely remove the influence of the non-uniformity of a static magnetic field and the non-linearity of a gradient magnetic field upon the image quality of an image formed by NMR imaging.
In an NMR imaging apparatus (hereinafter simply referred to as an "imaging apparatus"), the density distribution or relaxation time distribution of nuclear spin in a body to be inspected, is nondestructively detected by using the NMR phenomenon, and the cross section of a measuring target of the to-be-inspected body is reconstructed on the basis of the above distribution.
The projection-reconstruction method for forming the image of the cross section has been known. In this method, the projection of the spin density distribution or relaxation time distribution in the cross section is formed for various directions, and data thus obtained is processed to reconstruct the spin density distribution or relaxation time distribution in the cross section.
Further, another method for forming the image of the cross section, that is, the so-called direct Fourier imaging method has been known. In this method, the value of the Fourier transform of the spin density distribution or relaxation time distribution in the cross section is directly measured at points on a Cartesian Coordinate Matrix in the Fourier space. The direct Fourier imaging method includes, for example, the Fourier zeugmatography proposed by A. Kumar et al. (refer to Journal of Magnetic Resonance Vol. 13, 1975, pages 69 to 83) and the spin warp imaging proposed by W. A. Edelstein et al. (refer to Physics in Medicine & Biology Vol. 25, 1980, pages 751 to 756).
An imaging apparatus using the direct Fourier imaging method is required to have a static magnetic field having a uniform intensity distribution in a field of view, and a gradient magnetic field superposed on the static magnetic field for giving spatial information to a signal. However, in the case where the intensity distribution of the static magnetic field is non-uniform or the intensity of the gradient magnetic field varies non-linearly, there arises a problem that a geometrical distortion is produced on the image of a cross section.
Further, the non-uniformity of the static magnetic field and the non-linearity of the gradient magnetic field produce an error in the intensity of an image signal. Accordingly, in the case where the intensity distribution of the static magnetic field is extremely non-uniform or the intensity change of the gradient magnetic field is extremely non-linear, undesirable errors in the image intensities are produced.
Incidentally, an imaging apparatus capable of removing the infuence of the non-uniformity of a static magnetic field and the non-linearity of a gradient magnetic field upon an image obtained by the projection-reconstruction method, is disclosed in a Japanese Patent Application (Application No. 23547/1983) entitled "Imaging Apparatus using Nuclear Magnetic Resonance" and filed by the present applicant.